1. Field of Invention
This invention relates to variable power transmission devices for use with a crank-driven power train in which the crank lever passes through a prescribed arc of less than 180 degrees. More specifically, the present invention relates to a power transmission device useful as an automatic bicycle transmission.
2. Prior Art
The search for an efficient power transmission system for a bicycle has been both long and active. Ever since pedalled vehicles such as bicycles and tricycles were conceived, a strong interest toward finding improved power train systems has existed. Virtually all power train systems for pedalled vehicles include some form of primary drive wheel or sprocket coupled by a chain or pulley which rotates the axle of the rear wheel of the vehicle.
The most common transmission system for bicycles includes a rotary drive sprocket with attached pedals in opposing orientation. The pedals of the drive sprocket are rotated, which rotational energy is transferred by a chain or belt to a second drive sprocket of lesser dimension on th rear bicycle wheel. Multiple gearing systems have long existed for such bicycles. For example, U.S. Pat. No. 4,599,079 discloses the typical gearing construction for a ten or fifteen speed bicycle which utilizes a derailer to shift the chain to an appropriate sprocket for realizing a lower or higher gear ratio. The subject patent also discloses the use of an automatic shifter for the derailer, thereby freeing the bicyclist from shifting levers to shift gears. This system includes the use of a stabilizing hydraulic cylinder which is designed to inhibit clockwise rotation of the movable shifter arm during certain phases of operation This cylinder operates as a form of dampening means to control unwanted shifting between gears.
U.S. Pat. Nos. 3,779,099 and 3,906,807 by Trammell, Jr., disclose two additional approaches for improving power transfer in a bicycle transmission system. The second Trammell patent discloses a rotary drive system which automatically shifts pedal force between the crank arm and an advance-crank arm. The earlier Trammell patent suggests improvements to a vertically reciprocating pedal drive system. Although these respective patents provide improvement in applied moment arm to the drive sprocket, neither gives the broad range of shifting power as is provided by the multiple sprocket, derailer system as illustrated by Chappel in U.S. Pat. No. 4,599,079.
The vertically reciprocating pedal arrangement of the -099 patent of Trammell has likewise been within the prior art for many years. For example, in 1888, H. B. Smith Machine Company developed a tricycle which utilized the reciprocating vertical pedal motion to independently drive separate rear wheels. This tricycle is still on display in the Smithsonian Institute. Power transfer from the vertical pedal arrangement is generally accomplished through a roller clutch assembly, such as is disclosed in U.S. Pat. No. 4,019,230 by Pollard. This mechanical system allows the bicyclist to use downward pressure on the pedal as the power stroke, with the pedal being raised to its upright position by the power stroke of the other pedal. The use of such one-way bearings provides an efficient system for alternating power strokes between the feet of the bicyclist in an up and down motion, rather than the more traditional rotary motion normally associated with bicycles. Additional automatic transmission devices of interest are set forth in U.S. Pat. No. 4,127,038 which utilizes a front sprocket system, and U.S. Pat. No. 4,741,546 which utilizes multiple belt-driven expanding pulleys.
Despite the many years of innovation and change within the bicycle transmission art, none of the variety of improved transmission devices appears to have achieved any form of commercial success or public acceptance. In many instances, such devices and transmission systems are complex and expensive, and suffer from uncertain reliability. Accordingly, the majority of cyclists continue to manually shift between multiple sprockets to achieve various gear ratios ranging from low to high. Consequently, such cyclists continue to experience the frustration of manipulating a combination of levers to properly derail the drive chain between desired sprockets and to experience mechanical difficulties as chains and sprockets fail to mesh or otherwise malfunction.
What is needed, therefore, is a simple automatic transmission system which does not require multiple sprockets associated with a derailing system, and which automatically respond to the cyclist's changing needs of power versus speed, based on the changing topography of the land.